JP3673367B2 - 440 MPa class hot rolled steel sheet excellent in collision safety and method for producing the same - Google Patents

Description

これらの鋼を１１５０℃にて加熱し、熱間圧延を行い板厚２ｍｍの鋼板としたが、それぞれ表２〜１０に示す温度で熱延、巻取を行った。また、熱延終了温度から巻取温度までは、１０〜６０℃／ｓにて冷却を行った。その後、自動車の成形・焼付塗装を想定した予ひずみ、熱処理を行った。引張試験は静的に関してＪＩＳ５号試験片を用い、ＪＩＳ Ｚ ２２４１に従って行い、動的に関しては、板状試験片（Ｍａｓａａｋｉ Ｉｔａｂａｓｈｉ ａｎｄ Ｋｏｚｏ Ｋａｗａｔａ、Ｐｒｏｃｅｅｄｉｎｇ ｏｆ ＰＬＡＳＴＩＣＩＴＹ ’９５、ｐ．５２）を用い、ｏｎｅ ｂａｒ ｍｅｔｈｏｄ（材料、３６、（１９８７）、ｐ．１２９８）にて測定した。表２〜１０に、それぞれ、供試材の熱間圧延条件、高速変形前の予ひずみ量、熱処理温度および熱処理時間を示す。また、この条件下にて得られた、静的強度、式（１）右辺の値および得られた静動比も示す。
【００２７】
表２〜７は、それぞれ表１の鋼成分は本発明鋼の範囲内にある鋼種について、製造条件を変化させたものである。ここで、表４〜７の内での本発明範囲外のものは式（２）を満足しない条件にて巻取を行ったものである。ここで、製造条件が本発明範囲内ものは、得られた静動比が式（１）を満足するが、製造条件が式（２）を満たさない本発明範囲外のものは得られた静動比が式（１）を満足しないことがわかる。
【００２８】
表１０は表１中の鋼成分が本発明範囲外にあるものについて、製造条件を本発明範囲内で行ったものを示す。この比較鋼から、鋼成分が本発明範囲外になると式（１）の特性を満足しないことがわかる。これらの試験結果から、本発明によると高い静動比が得られることがわかる。
【００２９】
【表２】

【００３０】
【表３】

【００３４】
【表４】

【００３５】
【表５】

【００３６】
【表６】

【００３７】
【表７】

【００３８】
【発明の効果】
以上に述べたように、本発明によれば、静的強度に対し動的強度が高い鋼板の製造が可能となるため、本発明は、工業的に価値の高い発明であると言える。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength steel plate that achieves weight reduction and safety improvement of an automobile and a method for producing the same, and more particularly, to a 440 MPa class hot-rolled steel plate having excellent collision safety and a method for producing the same.
[0002]
[Prior art]
In recent years, demands for safety at the time of collision are becoming more and more strict from the viewpoint of improving the safety of automobiles. In order to achieve both reduction in weight of automobiles and improvement in impact absorption characteristics at the time of collision, it is suitable to use high-strength steel sheets as strength members for automobiles. Generally, high-strength steel sheets are more formable than low-strength steel sheets. This reduces the degree of freedom when designing automobiles. In addition, the strength (dynamic strength) at a strain rate (approximately 10 ³ (s ⁻¹ )) estimated to be dealt with at the time of collision is about 10 ⁻³ (s ⁻¹ ), which has been generally used conventionally. It does not necessarily correspond to the strength at a low strain rate (static strength). For example, when mild steel and high-strength steel are compared, it is known that the dynamic strength of high-strength steel is not as high as that of mild steel. For this reason, when the thickness is reduced based on the static strength value, the dynamic strength, which is an actual index of collision safety, is reduced.
[0003]
On the other hand, for example, in Japanese Patent Application Laid-Open No. 7-18370, from the viewpoint of safety of an actual automobile, not only mere strengthening but also Ti, Nb, Zr and V are added to form a solid solution C, N In addition, the invention of a steel sheet is disclosed in which a large amount of precipitates are precipitated and deformation resistance is increased when deformed at a high strain rate. From the same viewpoint, JP-A-7-18372 discloses a steel sheet invention in which the amount of retained austenite is 10% or more and the amount of C in the ferrite phase is reduced to 0.002% by weight or less. .
[0004]
[Problems to be solved by the invention]
However, since the steel sheet for automobiles is actually subjected to the steps of forming, painting, and subsequent painting and baking in the automobile manufacturing process, the above-described prior art that does not consider such history of the steel sheet for automobiles is a steel sheet at the time of collision. It cannot be said that the shock absorbing property is sufficiently evaluated and a solution is presented. That is, it is necessary to design a material by evaluating material characteristics based on dynamic strength after applying heat treatment equivalent to molding strain and paint baking.
[0005]
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a 440 MPa class hot-rolled steel sheet excellent in collision safety and a method for producing the same, which can advantageously solve the above problems and achieve weight reduction and safety improvement of an automobile. To do.
[0006]
[Means for Solving the Invention]
As a result of repeating various experiments and researches, the inventors of the present invention can improve the static ratio by appropriately adjusting the components of the steel material and applying pre-strain with the solid solution N remaining. And the present invention has been made. That is, the gist of the present invention is as follows.
[0007]
(1) By weight%
C: 0.01 to 0.08%,
Si: 1.5% or less,
Mn: 2.0% or less,
P: 0.05% or less,
S: 0.015% or less,
Al: 0.005 to 0.045% ,
N: 0.0041 to 0.015%
And the relationship between Al and N satisfies 2N—Al ≦ 0.012, the balance is Fe and inevitable impurities, and the maximum strength σS (when deformed at a strain rate of 10 ⁻³ (s ⁻¹ ) ( Mpa) and the maximum strength σD (MPa) when deformed at a strain rate of 10 ³ (s ⁻¹ ) after processing at a temperature T (° C.) and a time t (min) by giving a pre-strain E (%). The ratio σD / σS is
σD / σS ≧ 460 / σS + 0.35
+ 571E (t ^1/2 ) exp {−5000 / (T + 273)}
440 Mpa class hot rolled steel sheet excellent in collision safety, characterized by satisfying
[0008]
(2) By weight%
C: 0.01 to 0.08%,
Si: 1.5% or less,
Mn: 2.0% or less,
P: 0.05% or less,
S: 0.015% or less,
Al: 0.005 to 0.045% ,
N: 0.0041 to 0.015%
And the relationship between Al and N is 2N-Al ≦ 0.012
In which the balance is Fe and the inevitable impurities are hot-rolled so that the hot rolling finish temperature is Ar ₃ -50 ° C. or higher, and in the subsequent winding step, (2N—Al) < At 0.0016, the coiling temperature CT is set to CT <570-0.55 / {(2N-Al) -0.004}.
When (2N-Al) ≧ 0.0016, CT <800 ° C., and the maximum strength σS (Mpa) when deformed at a strain rate of 10 ⁻³ (s ⁻¹ ) The ratio σD / σS to the maximum strength σD (MPa) when deformed at a strain rate of 10 ³ (s ⁻¹ ) after applying strain E (%) at a temperature T (° C.) and time t (minutes) ,
σD / σS ≧ 460 / σS + 0.35
+ 571E (t ^1/2 ) exp {−5000 / (T + 273)}
The manufacturing method of the 440 MPa class hot-rolled steel plate excellent in the collision safety which satisfy | fills.
[0009]
In the present invention, the static ratio, which is a characteristic required for a steel sheet having excellent collision safety, is defined as the formability of steel, the dynamic strength after pre-straining and heat treatment corresponding to after baking, and the static strength before forming. The ratio to the target strength.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
[0011]
First, the reason why the steel components are limited will be described.
[0012]
C is an element necessary for obtaining strength and needs to be added a little. Moreover, it is set to 0.01% or more from the cost increase due to low C. However, a large amount of addition deteriorates the formability of the steel, so 0.08% or less.
[0013]
Si is a solid solution strengthening element. In the steel of the present invention, it is an element for adjusting the strength together with Mn. Moreover, addition is desirable because it promotes an increase in strength during high-speed deformation. However, if the addition amount increases, the moldability deteriorates and the static / dynamic ratio is lowered.
[0014]
Mn is a solid solution element like Si, and is an element for adjusting the strength in the steel of the present invention. However, if the addition amount increases, the moldability deteriorates and the static / dynamic ratio is lowered.
[0015]
P, like Si and Mn, is a solid solution strengthening element and makes it possible to adjust the strength of the steel sheet. Further, the crystal grain size is made fine to facilitate the remaining of the dissolved nitrogen. However, if the added amount increases, the grain boundary becomes weak, impact resistance deteriorates, and moldability deteriorates, so 0.05% or less. In particular, the addition of P is preferably 0.02% or less from the viewpoint of moldability.
[0016]
If the S content increases, the stretch flangeability, which is one of the important characteristics of the high-strength steel plate, deteriorates, so 0.015% or less.
[0017]
Al is added as a deoxidizer for making killed steel, and as an amount that satisfies this, 0.005% or more is necessary. However, if added in a large amount, precipitation of AlN is promoted, and a solid-state ratio that improves the static ratio is improved. In order to reduce dissolved nitrogen, the content is made 0.005 to 0.045%.
[0018]
N needs to be present in an amount sufficient to remain as dissolved nitrogen after winding in order to improve the static ratio. For this reason, the content of 0.0041 % or more is necessary. On the other hand, since addition of a large amount deteriorates moldability, it was made 0.015%.
[0019]
Moreover, the relationship between Al and N is defined as 2N-Al ≦ 0.012, but this formula defines the amount of dissolved nitrogen after winding, and if this is exceeded, the formability deteriorates significantly. The following.
[0020]
The properties of the steel are σD / σS ≧ 460 / σS + 0.35
+ 571E (t ^1/2 ) exp {−5000 / (T + 273)} (1)
However, in this equation, σD gives a pre-strain E (%) corresponding to molding in automobile manufacturing, and is subjected to a heat treatment at a temperature T (° C.) and a time t (minute) corresponding to paint baking. The maximum strength when deformed at a speed of 10 ³ (s ⁻¹ ), σS is the maximum strength when deformed at a strain rate of 10 ⁻³ (s ⁻¹ ). The characteristics of the steel material change in the strain introduced at the time of forming and the subsequent paint baking treatment, but this equation is a value when this change is taken into consideration.
[0021]
The steel having such a component is cast, and the obtained hot piece slab is directly or heated, or the cold piece is reheated to perform hot rolling. Hot rolling can be performed by either a normal hot process or a continuous hot rolling process in which slabs are joined and rolled in finish rolling.
[0022]
The rolling end temperature during hot rolling is Ar ₃ -50 ° C or higher. This is because if it is lower than Ar ₃ -50 ° C., the processed structure remains after rolling, and the formability of the steel sheet deteriorates.
[0023]
The coiling temperature CT is CT <570-0.55 / {(2N-Al) -0.004} at (2N-Al) <0.0016 (2)
When (2N-Al) ≧ 0.0016, CT <800 ° C. This regulates the coiling temperature in order to leave the solid solution nitrogen that improves the static ratio in the steel, and when the coil is wound at a temperature higher than this, the static ratio is hardly improved. To do. In particular, considering the cost merit in terms of equipment, the upper limit is the hot rolling end temperature, and the lower limit is normal temperature.
The present invention can be applied not only to a hot-rolled steel sheet but also to a surface-treated steel sheet using this as a raw material. In particular, among the steels of the present invention, steel having a Si concentration of 0.5% or less can be easily hot dip galvanized without harming the plating properties.
[0024]
【Example】
Hereinafter, the present invention will be specifically described by way of examples.
[0025]
Steels having various chemical components shown in Table 1 were cast with an actual machine and hot-rolled to make sample materials. Steel type numbers 1 to 4 and 8 to 11 are obtained by changing the amounts of Al and N and adjusting the strength using Si and Mn, and have components within the range of the steel of the present invention. Nos. 12 to 14 are comparative steels whose Si, Mn, and N concentrations are outside the range of the steel of the present invention.
[0026]
[Table 1]

These steels were heated at 1150 ° C. and hot-rolled to obtain steel plates having a thickness of 2 mm, but were hot rolled and wound at the temperatures shown in Tables 2 to 10, respectively. Moreover, it cooled at 10-60 degreeC / s from hot rolling completion temperature to coiling temperature. After that, pre-straining and heat treatment were performed assuming automobile molding and baking coating. Tensile tests are conducted according to JIS Z 2241 using JIS No. 5 test pieces for static, and plate tests (Masaaki Itabashi and Kozo Kawata, Proceeding of PLASTICITY '95, p. 52) are used for dynamic tests. Method (Material, 36, (1987), p. 1298). Tables 2 to 10 show the hot rolling conditions, the pre-strain amount before high speed deformation, the heat treatment temperature, and the heat treatment time of the test materials, respectively. In addition, the static strength, the value on the right side of Equation (1), and the obtained static / dynamic ratio obtained under these conditions are also shown.
[0027]
In Tables 2 to 7 , the steel components in Table 1 are obtained by changing the production conditions for the steel types in the range of the steel of the present invention. Here, those outside the scope of the present invention in Tables 4 to 7 are those wound under conditions that do not satisfy the formula (2). Here, when the manufacturing conditions are within the range of the present invention, the obtained static ratio satisfies the formula (1), but when the manufacturing conditions do not satisfy the formula (2), those outside the range of the present invention are obtained. It can be seen that the dynamic ratio does not satisfy Equation (1).
[0028]
Table 10 shows that the steel components in Table 1 were outside the scope of the present invention and the production conditions were within the scope of the present invention. From this comparative steel, it can be seen that when the steel component falls outside the scope of the present invention, the characteristic of the formula (1) is not satisfied. From these test results, it can be seen that a high static ratio can be obtained according to the present invention.
[0029]
[Table 2]

[0030]
[Table 3]

[0034]
[Table 4]

[0035]
[Table 5]

[0036]
[Table 6]

[0037]
[Table 7]

[0038]
【The invention's effect】
As described above, according to the present invention, it is possible to produce a steel plate having a high dynamic strength with respect to a static strength, and therefore it can be said that the present invention is an industrially valuable invention.

Claims (2)

% By weight
C: 0.01 to 0.08%,
Si: 1.5% or less,
Mn: 2.0% or less,
P: 0.05% or less,
S: 0.015% or less,
Al: 0.005 to 0.045% ,
N: 0.0041 to 0.015%
And the relationship between Al and N is 2N-Al ≦ 0.012
With the balance consisting of Fe and unavoidable impurities, the maximum strength σS (Mpa) when deformed at a strain rate of 10 ⁻³ (s ⁻¹ ), and the prestrain E (%) and the temperature T (° C. ), The ratio σD / σS to the maximum strength σD (MPa) when deformed at a strain rate of 10 ³ (s ⁻¹ ) after processing at time t (minutes),
σD / σS ≧ 460 / σS + 0.35 + 571E (t ^1/2 ) exp {−5000 / (T + 273)}
440 Mpa class hot rolled steel sheet excellent in collision safety, characterized by satisfying % By weight
C: 0.01 to 0.08%,
Si: 1.5% or less,
Mn: 2.0% or less,
P: 0.05% or less,
S: 0.015% or less,
Al: 0.005 to 0.045% ,
N: 0.0041 to 0.015%
And the relationship between Al and N is 2N-Al ≦ 0.012
In which the balance is Fe and the inevitable impurities are hot-rolled so that the hot rolling finish temperature is Ar ₃ -50 ° C. or higher, and in the subsequent winding step, (2N—Al) < At 0.0016, the coiling temperature CT is set to CT <570-0.55 / {(2N-Al) -0.004}.
When (2N-Al) ≧ 0.0016, CT <800 ° C., and the maximum strength σS (Mpa) when deformed at a strain rate of 10 ⁻³ (s ⁻¹ ) The ratio σD / σS to the maximum strength σD (MPa) when deformed at a strain rate of 10 ³ (s ⁻¹ ) after applying strain E (%) at a temperature T (° C.) and time t (minutes) ,
σD / σS ≧ 460 / σS + 0.35 + 571E (t ^1/2 ) exp {−5000 / (T + 273)}
The manufacturing method of the 440 MPa class hot-rolled steel plate excellent in the collision safety which satisfy | fills.